The Android app is made using MIT App Inventor which is
an easy way to make Android apps.
http://appinventor.mit.edu/explore/ . The app has buttons to
connect and disconnect the Bluetooth to the phone and 2 displays of
voltage and frequency. The .apk file is the file that installs
manually the app to your phone. The .aia file can be imported after you
install the MIT App Inventor.The Arduino generates an accurate 1 second time base
for the counter by cascading timer0 and timer2. The link between digital
inputs 3 and 4 connects the output of timer2, 250 Hz, to input of
timer0. The software waits for the output of timer0 to go positive to
start the count of frequency input to timer1. Timer1 is a 16 bits timer,
it overflows at the count of 32768, that in turn advances overFlow
register. At the end of the 1 second the 16 bit register is recorded.
The Arduino then sends to the Bluetooth 5 bytes of data in serial RS232
protocol.HC-05 is pre-set to code 1234 and name HC-05. The serial
RS232 is pre-set to 9600,n,8,1.

ARDUINO
CONTROLED BY ANDROID DEVICE VIA USB

The Android app is made using MIT App Inventor which is
an easy way to make Android apps.
http://appinventor.mit.edu/explore/ . The Arduino is connected
to the Android device using OTG cable, the device also powers the
Arduino. The communication is transferred in serial protocol. The app has buttons to
connect and disconnect the USB to the phone. There are also 3 LEDS that
indicate the Arduino inputs state and 4 tick boxes to control the
outputs of the Arduino.The .apk file is the file that installs
manually the app to your device. If you wish to change the app the .aia file can be imported after you
install the MIT App Inventor.

ARDUINO
WIRELESS REMOTE FOR ANDROID

The Android app is written in BASIC!
http://mougino.free.fr/rfo-basic/index.html#QuickAPK . The app has
connects the Bluetooth. The .apk file is the file that installs
manually the app to your phone. The .bas file can be imported after you
install the RFO BASIC app to your PC and or your Arduino device.The Arduino
sends via the Bluetoth serial value representing the switch that was
pressed. The BASIC! app on the Android phone does these operations. switch 5.
Starts the Camera 6. Starts Calculator 7. Starts the browser and
logs to this website 8. Starts gmailTo change the action of the
switches in BASIC! it requires to enter the Activity Package and
class of the app.

PIC16F690 FREQUENCY COUNTER FOR ANDROID

This project is for the Android app and HC-05 as above.
The PIC counts the frequency over 1 second period generated by CCP1 and
Timer1. Input is counted by Timer0 that can count up to 4 MHz. After
each count period 2 bytes of ADC and 3 bytes of the counter are
transmitted to the HC-05 Bluetooth module.